Granite samples collected from the Mekong River Valley reveal that the river’s path was incised roughly 17 million years ago, most likely by increased erosion from monsoon precipitation.

Two men fish in the Mekong River. Credit: iStock.com/prachanart

By Katherine Kornei  14 hours ago

Estimates of the age of this mighty river, one of the world’s largest, are wildly inconsistent.The Mekong River winds more than 4,000 kilometers through Southeast Asia, where millions of people rely on it for food, income, and transportation. But estimates of the age of this mighty river, one of the world’s largest, are wildly inconsistent.

Now scientists have age dated samples of bedrock granite from the Mekong River Valley and have shown that the river likely incised its path from its headwaters through the Tibetan Plateau roughly 17 million years ago. This timing is coincident with a warm period of heavier-than-usual monsoon rains, which triggered increased erosion that formed the Mekong River, the researchers proposed.

Important but Overlooked

Despite the importance of the Mekong River to many people in Southeast Asia, the history of its formation “has largely been overlooked,” said Peter Clift, a geologist at Louisiana State University in Baton Rouge who was not involved in the research.

That knowledge gap piqued the curiosity of Junsheng Nie, a geologist at Lanzhou University in Lanzhou, China. Prior research showed that the Tibetan highlands began to rise 30–40 million years ago, creating the elevation differences needed for a river to cut through surrounding rock. But when exactly did the river incise?

This method doesn’t date the rocks themselves, but rather the time when they were raised near Earth’s surface.To learn more, Nie and his team collected six samples of granite from sites along the Mekong River spanning 1,300 meters in elevation. The researchers applied an age-dating technique to the samples to estimate when, roughly, the river incised its path. This method doesn’t date the rocks themselves, Nie noted, but rather the time when they were raised near Earth’s surface, which would cause the river to erode the terrain.

The technique involves measuring two heavier “parent” elements (uranium and thorium) and one lighter “daughter” element (helium) produced when uranium and thorium decay. By knowing the elements’ half-lives and measuring the concentrations of the parent elements and the daughter element, Nie and his team calculated how long ago the rocks were brought near the surface.

This technique works because it hinges on the rocks containing helium, said Nie. This light element tends to escape from rocks when their surrounding environment is too hot, like the conditions found deep within Earth. However, when rocks are uplifted, they start to retain helium. As a result, ratios of helium to uranium and thorium in a rock help to give a time stamp of when the rock uplifted.

Increased Precipitation

Nie and his team found that the rocks were raised near Earth’s surface roughly 17 million years ago. Interestingly, this age corresponds to an interval known as the Middle Miocene Climate Optimum, the researchers noted.

During this interval, which spanned roughly 14 to 17 million years ago, Earth’s climate went through a warm phase with high levels of carbon dioxide. When the climate is warmer, it’s more dynamic, said Nie. “There’s more water evaporating from the ocean…which can cause an increase in monsoon precipitation.” This increased precipitation, in turn, increases erosion as water sluices off highlands.

Prior studies have indicated that precipitation rates during the Middle Miocene Climate Optimum were about double that of current conditions. The researchers then asked, Could this have been enough to cut a river?

“The Mekong River in its modern extent probably did not exist prior to 17 million years ago.”To answer this question, they turned to a computer model of a simulated landscape devoid of rivers that stretched from Tibet. Using the higher precipitation rates, the scientists found that their models reproduced a kilometer-scale incision in the rock, similar to the modern-day Mekong River Valley.

“The Mekong River in its modern extent probably did not exist prior to 17 million years ago,” the researchers concluded in their paper published last week in Nature Geoscience.

Nie and his colleagues plan to return to the Mekong River to collect more rocks and determine when they were brought near the surface. By better pinpointing the age of this river, the researchers hope to refine techniques that will allow them to better constrain the ages of other major rivers nearby—like the Yangtze, Salween, and Red rivers.

—Katherine Kornei (email: hobbies4kk@gmail.com; @katherinekornei), Freelance Science Journalist